Vehicle underbody structure and vehicle body

ABSTRACT

The vehicle underbody structure ( 20 ) includes a floor panel ( 24 ) and an underbody reinforcement element ( 26 ), the underbody reinforcement element ( 26 ) including at least a front transversal beam ( 112   a,    112   b ) and a rear transversal beam ( 114 ), the transversal beams ( 112   a,    112   b,    114 ) extending parallel to each other and being attached to the floor panel ( 24 ). The front transversal beam ( 112   a,    112   b ) and the rear transversal beam ( 114 ) are made of a press hardening steel having a tensile strength higher than or equal to 1300 MPa.

The invention relates to a vehicle underbody structure, comprising afloor panel and an underbody reinforcement element, said underbodyreinforcement element comprising at least a front transversal beam and arear transversal beam, said transversal beams extending parallel to eachother and being attached to the floor panel, characterized in that thefront transversal beam and the rear transversal beam are made of a presshardening steel having a tensile strength higher than or equal to 1300MPa.

BACKGROUND

Conventionally, vehicles comprise reinforcement elements destined toprotect the vehicle occupants in case of an impact, in particular a sideimpact, a front impact or a roof crush during a rollover accident, bylimiting intrusions of any kind in the vehicle passenger compartment.

The reinforcement elements comprise, on each side of the vehicle, a bodyside structure forming a door frame, which is conventionally made ofsteel, and an underbody reinforcement element forming with a floor panelthe vehicle underbody structure.

Reducing the overall weight of the vehicle is very desirable in order toreduce the energy consumption, so as to meet the future environmentalrequirements. It is thus desirable to reduce the thickness of the partsforming the vehicle, in particular the reinforcement elements, includingthe underbody reinforcement element.

However, simply reducing the thickness of the reinforcement elementsgenerally leads to a degraded protection of the vehicle occupantsagainst intrusions.

Steels having better mechanical properties such as yield strength ortensile strength may be used. Indeed, such steels provide an enhancedprotection that lower grade steels for a given thickness. However, suchsteels generally have a lower formability.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a vehicle underbodystructure comprising an underbody reinforcement element having reducedthickness and weights together with a satisfactory protection to thevehicle occupants.

To that end, the invention relates to a vehicle underbody structure ofthe aforementioned type, characterized in that the front transversalbeam and the rear transversal beam are made of a press hardening steelhaving a tensile strength higher than or equal to 1300 MPa.

The use of a press hardening steel having a tensile strength higher thanor equal to 1300 MPa provides an improved resistance to impacts, inparticular side impacts, without requiring thickening the parts formingthe underbody reinforcement element, and thus without increasing theweight of the vehicle underbody structure.

According to other advantageous aspects of the invention, the vehicleunderbody structure comprises one or more of the following features,considered alone or according to any technically possible combination:

-   -   the underbody reinforcement element further comprises an        intermediate transversal beam extending between and parallel to        the front transversal beam and the rear transversal beam, said        intermediate transversal beam being made of a press hardening        steel having a tensile strength higher than or equal to 1300        MPa;    -   the underbody reinforcement element comprises at least one        longitudinal beam extending from the front transversal beam to        the rear transversal beam, said longitudinal beam being made of        a press hardening steel having a tensile strength higher than or        equal to 1300 MPa;    -   the floor panel comprises a longitudinal floor tunnel forming a        recess in the floor panel, the underbody reinforcement element        comprising two front transversal beams extending transversally        on either side of the floor tunnel;    -   the underbody reinforcement element comprises two longitudinal        beams extending on either side of the floor tunnel and each        joining one of the front transversal beam to the rear        transversal beam;    -   the underbody reinforcement element comprises a tunnel        transversal beam extending from one longitudinal beam to the        other longitudinal beam and across the floor tunnel, said tunnel        transversal beam being made of a press hardening steel having a        tensile strength higher than or equal to 1300 MPa;    -   the underbody reinforcement element comprises a single rear        transversal beam extending transversally from a transversal end        of the floor panel to the other transversal end of said floor        panel at a longitudinal end of the floor tunnel;    -   the press hardening steel is Usibor®;    -   the underbody reinforcement element forms an opened profile,        said opened profile being closed by the floor panel;    -   the material of the floor panel is different than the material        of the underbody reinforcement element;    -   the vehicle underbody structure further comprises at least one        connection element of the vehicle underbody structure to a        pick-up chassis, said connection element being attached to one        of the transversal beam of the underbody reinforcement element.

The invention also relates to a vehicle body comprising at least avehicle door frame and a vehicle underbody structure according to theinvention, wherein the vehicle door frame comprises at least a frontpillar, a center pillar and a side rail joining the front pillar and thecenter pillar, the front pillar being attached to the front transversalbeam of the underbody reinforcement element and the center pillar beingattached to the rear transversal beam of the underbody reinforcementelement.

According to other advantageous aspects of the invention, the vehiclebody comprises one or more of the following features, considered aloneor according to any technically possible combination:

-   -   the door frame further comprises a lowerside sill joining the        front pillar to the center pillar, the transversal beams of the        underbody reinforcement element being welded to said lowerside        sill;    -   the intermediate transversal beam is attached to the lowerside        sill of the door frame;    -   the lowerside sill comprises an outer lowerside sill part and an        inner lowerside sill part, said transversal beams of the        underbody reinforcement element being welded to said inner        lowerside sill part;    -   said inner lowerside sill part is made of a steel having a fully        martensitic microstructure, said steel having a tensile strength        higher than or equal to 1700 MPa.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will be better understoodfrom a reading of the following description, given with reference to theappended drawings, in which:

FIG. 1 is a perspective view of a vehicle body according to a particularembodiment;

FIG. 2 shows an exploded perspective view of a vehicle door frame of thevehicle body of FIG. 1;

FIG. 3 is a bottom view of the underbody structure of the vehicle bodyof FIG. 1;

FIG. 4 shows a cross-section along line IV-IV of FIG. 3 of an assemblyof the vehicle door frame and the underbody structure.

DETAILED DESCRIPTION

In the following description, the terms inner, outer, front, rear,transversal, longitudinal, vertical and horizontal are construed withreference to the usual orientation of the illustrated elements, parts orstructures when assembled on a vehicle structure.

A vehicle body 10 according to an embodiment of the invention isillustrated on FIG. 1. The vehicle body 10 is a body of a vehicle of thebody-on-frame type, for example a pick-up truck. A vehicle of this typecomprises a chassis which is separate from the body of the vehicle.

The vehicle body 10 comprises a vehicle underbody structure 20, and, oneither side of the vehicle underbody structure 20, a vehicle door frame22.

The vehicle underbody structure 20 includes a floor panel 24 and anunderbody reinforcement element 26, shown on FIG. 3. The vehicleunderbody structure 20 is destined to be connected to the chassis of thevehicle, as described in further details herebelow.

The vehicle door frame 22 and each underbody reinforcement element 26together form at least part of a vehicle reinforcement structure 30.

The vehicle door frame 22 comprises a side rail 34, a center pillar 36and a front pillar 38. In the illustrated embodiment, the vehicle doorframe 22 further comprises a lowerside sill 40.

The side rail 34 extends substantially horizontally along a longitudinaldirection between a front end 34 a and a rear end 34 b.

The front pillar 38 extends downward from the front end 34 a of the siderail 34 along an overall substantially vertical direction. The frontpillar 38 comprises an upper front pillar section 42 and a lower frontpillar section 44.

The upper front pillar section 42 extends frontward and downward fromthe front end 34 a of the side rail 34 in an oblique direction. Theupper front pillar section 42 thus extends between an upper end 42 acontiguous to the front end 34 a of the side rail 34, and a lower end 42b.

The lower front pillar section 44 extends substantially vertically fromthe lower end 42 b of the upper front pillar section 42. The lower frontpillar section 44 extends between an upper end 44 a, contiguous to thelower end 42 b of the upper front pillar section 42, and a lower end 44b.

The center pillar 36 extends downward from the side rail 34 along asubstantially vertical direction. In the illustrated example, the centerpillar 36 extends from an intermediate section of the side rail 34comprised between the front end 34 a and the rear end 34 b of the siderail 34. The center pillar 36 thus extends between an upper end 36 a,contiguous to the intermediate section of the side rail 34, and a lowerend 36 b.

Thus, the side rail 34 joins the upper ends of the front pillar 38 andthe center pillar 36.

The lowerside sill 40 extends substantially along a longitudinaldirection between a front end 40 a and a rear end 40 b.

The lower end of the front pillar 38 is contiguous to the front end 40 aof the lowerside sill 40, and the lower end 36 b of the center pillar 36is contiguous to an intermediate section of the lowerside sill 40comprised between the front end 40 a and the rear end 40 b of thelowerside sill 40. Thus, the lowerside sill 40 joins the lower ends ofthe front pillar 38 and the center pillar 36.

Thus, the front pillar 38, the center pillar 36, the side rail 34 andthe lowerside sill 40 define a door opening. Indeed, the vehicle doorframe 22 is destined to receive a vehicle door, which, when closed,fills the door opening.

As illustrated on FIG. 2, the vehicle door frame 22 is formed by theassembly of a vehicle door frame outer reinforcement element 50 and of avehicle door frame inner reinforcement element 52.

The vehicle door frame inner reinforcement element 52 and the vehicledoor frame outer reinforcement element 50 each are three-dimensionalelements.

The vehicle door frame inner reinforcement element 52 comprises innerreinforcement parts including an inner side rail part 64, an innercenter pillar part 66 and an inner front pillar part 68. The innerreinforcement parts further include a roof reinforcement part 70.

Similarly to the side rail 34, the inner side rail part 64 extendssubstantially horizontally along a longitudinal direction between afront end 64 a and a rear end 64 b.

Similarly to the center pillar 36, the inner center pillar part 66extends downward from the inner side rail part 64 along a substantiallyvertical direction. In the illustrated example, the inner center pillarpart 66 extends from an intermediate section of the inner side rail part64 comprised between the front end 64 a and the rear end 64 b of theinner side rail part 64. The inner center pillar part 66 thus extendsbetween an upper end 66 a contiguous to the intermediate section of theinner side rail part 64, and a lower end 66 b.

The inner front pillar part 68 extends frontward and downward from thefront end 64 a of the inner side rail part 64 in an oblique direction.The inner front pillar part 68 thus extends between an upper end 68 acontiguous to the front end of the side rail 34, and a lower end 68 b.Preferably, the lower end 68 b is comprised in a horizontal plane higherthan the horizontal plane comprising the lower end of the front pillar38, i.e. the inner front pillar part 68 does not extend along the wholelength of the front pillar 38, and for example only extends along thelength of the upper front pillar section 42.

Thus, the inner side rail part 64 joins the inner front pillar part 68and the inner center pillar part 66.

The roof reinforcement part 70 extends in a substantially horizontalplane inwardly from the front end 64 a of the inner side rail part 64,in a direction substantially perpendicular to the inner front pillarpart 68 and to the inner side rail part 64.

The roof reinforcement part 70 is destined to support a roof front railelement.

The roof reinforcement part 70 is for example made integral with theinner side rail part 64.

At least part of the inner reinforcement parts are made of a presshardened steel. For example, all the inner reinforcement parts are madeof a press hardened steel. The inner reinforcement parts may be made ofdifferent press hardened steels.

Preferably, the press hardened steel has a tensile strength greater thanor equal to 1300 MPa.

For example, the press-hardened steel has a composition comprising, in %weight, 0.10%≦C≦0.5%, 0.5%≦Mn≦3%, 0.1%≦Si≦1%, 0.01%≦Cr≦1%, Ti≦0.2%,Al≦0.1%, S≦0.05%, P≦0.1%, 0.0005%≦B≦0.010%, the remainder consisting ofiron and unavoidable impurities resulting from the production.

The press hardened steel is for example Usibor®, in particularUsibor®1500.

The steel may be coated or uncoated, for example galvannealed orgalvanized by any suitable process such as hot dip coating,electrodeposition, vacuum coating.

In particular, the press hardened steel has preferably a structureconsisting essentially of ferrite and perlite before the steel is hotstamped, and a structure consisting essentially of martensite after hotstamping.

The vehicle door frame inner reinforcement element 52 has an overallthickness, defined as the smallest dimension of the vehicle door frameinner reinforcement element 52, for example comprised between 0.7 mm and1.3 mm.

Preferably, at least one of the inner reinforcement parts has athickness different from the thickness of the other inner reinforcementparts. For example, the thicknesses of the inner side rail part 64, theinner center pillar part 66 and the inner front pillar part 68 differone from each other.

Each inner reinforcement part has a thickness adapted to the finalposition of the part in the vehicle and to the resistance to beachieved.

For example, the inner side rail part 64, together with the roofreinforcement part 70, has a thickness substantially equal to 1 mm. Theinner front pillar part 68 has a thickness substantially equal to 0.9mm. The inner center pillar part 66 has a thickness substantially equalto 1.1 mm.

The vehicle door frame outer reinforcement element 50 comprises outerreinforcement parts including an outer side rail part 74, an outercenter pillar part 76 and an outer front pillar part 78. The outerreinforcement parts further include an outer lowerside sill part 80.

Similarly to the side rail 34, the outer side rail part 74 extendssubstantially horizontally along a longitudinal direction between afront end 74 a and a rear end 74 b.

Similarly to the center pillar 36, the outer center pillar part 76extends downward from the outer side rail part 74 along a substantiallyvertical direction. In the illustrated example, the outer center pillarpart 76 extends from an intermediate section of the outer side rail part74 comprised between the front end 74 a and the rear end 74 b of theouter side rail part 74. The outer center pillar part 76 thus extendsbetween an upper end 76 a, contiguous to the intermediate section of theouter side rail part 74, and a lower end 76 b.

Similarly to the front pillar 38, the outer front pillar part 78 extendsdownward from the front end 74 a of the outer side rail part 74 along anoverall substantially vertical direction. The outer front pillar part 78comprises an outer upper front pillar part 82 and an outer lower frontpillar part 84.

The outer upper front pillar part 82 extends frontward and downward fromthe front end 74 a of the outer side rail part 74 in an obliquedirection. The outer upper front pillar part 82 thus extends between anupper end 82 a contiguous to the front end 74 a of the outer side railpart 74, which is the upper end of the outer front pillar part 78, and alower end 82 b.

The outer lower front pillar part 84 extends substantially verticallyfrom the lower end of the outer upper front pillar part 82. The outerlower front pillar part 84 extends between an upper end 84 a, contiguousto the lower end 82 b of the outer upper front pillar part 82, and alower end 84 b, which is the lower end of the outer front pillar part78.

Thus, the outer side rail part 74 joins the outer front pillar part 78and the outer center pillar part 76.

Similarly to the lowerside sill 40, the outer lowerside sill part 80extends substantially along a longitudinal direction between a front end80 a and a rear end 80 b.

The lower end 84 b of the outer front pillar part 78 is contiguous tothe front end 76 a of the outer lowerside sill part 80, and the lowerend 76 b of the outer center pillar part 76 is contiguous to anintermediate section of the outer lowerside sill part 80 comprisedbetween the front end 80 a and the rear end 80 b of the outer lowersidesill part 80. Thus, the outer lowerside sill part 80 joins the outerlower front pillar part 84 to the outer center pillar part 76.

At least part of the outer reinforcement parts are made of a presshardened steel. For example, all the outer reinforcement parts are madeof a press hardened steel.

Preferably, the press hardened steel has a tensile strength higher thanor equal to 1300 MPa.

For example, the press-hardened steel has a composition comprising, in %weight, 0.10%≦C≦0.5%, 0.5%≦Mn≦3%, 0.1%≦Si≦1%, 0.01%≦Cr≦1%, Ti≦0.2%,Al≦0.1%, S≦0.05%, P≦0.1%, 0.0005%≦B≦0.010%, the remainder consisting ofiron and unavoidable impurities resulting from the production.

The press hardened steel is for example Usibor®, in particularUsibor®1500.

The steel may be coated or uncoated, for example galvannealed orgalvanized by any suitable process such as hot dip coating,electrodeposition, vacuum coating.

The outer reinforcement parts may be made of different press hardenedsteels. For example, the outer lowerside sill part 80 may be made of apress hardened steel different from the other outer parts. Inparticular, the outer lowerside sill part 80 may be made of Ductibor®,and the other outer reinforcement parts are made of Usibor®.

However, the outer reinforcement parts are preferably all made of thesame press hardened steel, so that when subjected to an external stress,the distribution of the deformation is homogeneous within the outerreinforcement element 50.

In particular, the press hardened steel has preferably a structureconsisting essentially of ferrite and perlite before the steel is hotstamped, and a structure consisting essentially of martensite after hotstamping.

The door frame outer reinforcement element 50 has an overall thickness,defined as the smallest dimension of the door frame outer reinforcementelement 50, for example comprised between 0.8 mm and 2.5 mm.

Preferably, at least one of the outer reinforcement parts has athickness different from the thickness of the other outer reinforcementparts. For example, the thicknesses of the outer side rail part 74, theouter center pillar part 76 and the outer front pillar part 78 differone from each other.

Each outer part has a thickness adapted to the final position of thepart in the vehicle and to the resistance to be achieved.

For example, the outer side rail part 74 has a thickness substantiallyequal to 1.1 mm, the outer upper front pillar part 82 has a thicknesssubstantially equal to 0.9 mm, and the outer lower front pillar part 84has a thickness substantially equal to 1.1 mm. According to thisexample, the outer center pillar part 76 has a thickness substantiallyequal to 2 mm and the outer lowerside sill part 80 has a thicknesssubstantially equal to 1.2 mm.

The door frame outer reinforcement element 50 and the door frame innerreinforcement element 52 have complementary shapes such that, onceassembled, the door frame outer reinforcement element 50 and the doorframe inner reinforcement element 52 form the vehicle door frame 22.

In particular, the side rail 34 is formed by the assembly of the innerside rail part 64 and the outer side rail part 74, and the center pillar36 is formed by the assembly of the inner center pillar part 66 and theouter center pillar part 76.

Furthermore, the front pillar 38 is formed by the assembly of the innerfront pillar part 68 and the outer front pillar part 78. Morespecifically, the upper front pillar section 42 is formed by theassembly of the inner front pillar part 68 and the outer upper frontpillar part 82.

In the illustrated example, the lower front pillar section 44 is formedby the outer lower front pillar part 84.

The vehicle door frame outer reinforcement element 50 and the vehicledoor frame inner reinforcement element 52 are shaped as complementaryopened profiles such that a part of the front pillar 38, the centerpillar 36 and the side rail 34 of the vehicle door frame 22 each have ahollow closed section.

In particular, the outer reinforcement parts have an open cross-section.The open cross-section comprises at least a bottom segment and two wallsegments extending from either ends of the bottom segment.

A method for producing the vehicle door frame 22 will now be described.

The production of the vehicle door frame 22 comprises producing the doorframe inner reinforcement element 52, producing the door frame outerreinforcement element 50, and assembling the door frame innerreinforcement element 52 to the door frame outer reinforcement element50.

The vehicle door frame inner reinforcement element 52 is produced by hotstamping an inner reinforcement blank, which is itself formed byassembling several blanks.

The production of the vehicle door frame inner reinforcement element 52thus comprises a step of formation of a substantially planar door frameinner reinforcement blank. The shape of the door frame innerreinforcement blank is adapted so that it may be hot stamped to form theinner reinforcement element having the desired shape.

The inner reinforcement blank is preferably a tailor welded blank.

The formation of the inner reinforcement blank comprises providing aninner center pillar blank, an inner front pillar blank and an inner siderail blank, said inner blanks being substantially planar. Preferably,the formation of the inner reinforcement blank further comprisesproviding a roof reinforcement blank made integral with the inner siderail blank.

The shapes and thicknesses of the inner blanks are adapted so that, oncehot stamped, the inner center pillar blank, the inner front pillarblank, the inner side rail blank and the roof reinforcement blank mayform the inner center pillar part 66, the inner front pillar part 68,the inner side rail part 64 and the roof reinforcement part 70respectively.

The inner blanks are for example obtained by cutting steel sheets, forexample sheets made of a press hardening steel such as Usibor®, to thedesired shapes.

The inner center pillar blank and the inner front pillar blank are thenassembled to the inner side rail blank to form the inner reinforcementblank.

In particular, an upper end of the inner front pillar blank is assembledto a front end of the inner side rail blank and an upper end of theinner center pillar blank is assembled to an intermediate section of theinner side rail blank.

Preferably, the inner blanks are assembled by means of welding, morepreferably by laser welding, such that the inner blanks are joinedtogether by continuous weld lines.

In particular, the inner center pillar blank and the inner front pillarblank are joined to the inner side rail blank by continuous weld lines.

The production of the vehicle door frame inner reinforcement element 52then comprises a step of hot stamping the door frame inner reinforcementblank to shape the three-dimensional vehicle door frame innerreinforcement element 52.

If the inner blanks are made of a press hardening steel, the hotstamping results in a hardening of the steel.

In particular, as stated above, the press hardened steel has preferablya structure consisting essentially of ferrite and perlite before thesteel is hot stamped, and a structure consisting essentially ofmartensite after hot stamping and quenching.

Similarly, the door frame outer reinforcement element 50 is produced byhot stamping an outer reinforcement blank, which is itself formed byassembling several blanks.

The production of the door frame outer reinforcement element 50 thuscomprises a step of formation of a substantially planar door frame outerreinforcement blank. The shape of the door frame outer reinforcementblank is adapted so that it may be hot stamped to form the outerreinforcement element having the desired shape.

The outer reinforcement blank is preferably a tailor welded blank.

The formation of the outer reinforcement blank comprises providing anouter center pillar blank, an outer upper front pillar blank, an outerlower front pillar blank, an outer side rail blank, and an outerlowerside sill blank, said outer blanks being substantially planar.

The shapes and thicknesses of the outer blanks are adapted so that, oncehot stamped, the outer center pillar blank, the outer upper front pillarblank, the outer lower front pillar blank, the outer side rail blank andthe outer lowerside sill blank may form the outer center pillar part 76,the outer upper front pillar part 82, the outer lower front pillar part84, the outer side rail part 74 and the outer lowerside sill part 80respectively.

The outer blanks are for example obtained by cutting steel sheets, forexample sheets made of a press hardening steel such as Usibor®, to thedesired shapes.

The outer center pillar blank and the outer upper front pillar blank arethen assembled to the outer side rail blank, the outer lower frontpillar blank is assembled to the outer upper front pillar blank, and theouter lowerside sill blank is assembled to the outer lower front pillarblank and to the outer center pillar blank, so as to form the outerreinforcement blank.

In particular, an upper end of the outer upper front pillar blank isassembled to a front end of the outer side rail blank and an upper endof the outer center pillar blank is assembled to an intermediate sectionof the outer side rail blank. Furthermore, a lower end of the outerupper front pillar blank is assembled to an upper end of the outer lowerfront pillar blank, a lower end of the outer lower front pillar blank isassembled to a front end of the outer lowerside sill blank, and a lowerend of the outer center pillar blank is assembled to an intermediatesection of the outer lowerside sill blank.

Preferably, the outer blanks are assembled by means of welding, morepreferably by laser welding, such that the outer blanks are joinedtogether by continuous weld lines.

The production of the door frame outer reinforcement element 50 thencomprises a step of hot stamping the door frame outer reinforcementblank to shape the three-dimensional vehicle door frame outerreinforcement element 50.

If the outer blanks are made of a press hardening steel, the hotstamping results in a hardening of the steel.

The door frame inner reinforcement element 52 and the door frame outerreinforcement element 50 are then assembled, for example by welding.

The use of a press hardening steel for the inner and outer blanks thusprovides both a good formability for the blanks so that the door frameinner and outer reinforcement blanks may be hot stamped to shape thevehicle door frame inner and outer reinforcement elements withoutobtaining necking or thickening of the steel, and a ultra high strengthfor the vehicle door frame inner and outer reinforcement elements oncehot stamped.

Assembling the inner blanks (respectively the outer blanks) to form thedoor frame inner reinforcement blank (respectively the door frame outerreinforcement blank) before hot stamping the door frame innerreinforcement blank (respectively the door frame outer reinforcementblank) allows the use of a single press for shaping the whole vehicledoor frame inner reinforcement element 52 (respectively the whole doorframe outer reinforcement element 50), which reduces the cost of theproduction of the vehicle door frame inner 52 and outer 50 reinforcementelements.

Moreover, the formation of the door frame inner reinforcement blank(respectively the door frame outer reinforcement blank) by assemblingseveral inner blanks (respectively several outer blanks) allows having avarying thickness between the different parts of the vehicle door frameinner reinforcement element 52 (respectively the door frame outerreinforcement element 50), and further allows the reduction of materialuse thanks to improved nesting.

Furthermore, assembling the inner blanks (respectively the outer blanks)to form the door frame inner reinforcement blank (respectively the doorframe outer reinforcement blank) before hot stamping the door frameinner reinforcement blank (respectively the door frame outerreinforcement blank) allows using laser welding to assemble the innerblanks (respectively the outer blanks), instead of spot welding. Laserwelding provides a continuous weld line between the blanks, andconsequently provides a better resistance, and thus a better crashresistance, than spot welding.

Referring now to FIG. 3, the vehicle underbody structure 20 includes afloor panel 24 and an underbody reinforcement element 26.

The floor panel 24 generally extends along a horizontal plane. The floorpanel 24 extends longitudinally between a front side 24 a and a rearside 24 b, and transversally between a right side 24 c and a left side24 d.

The floor panel 24 comprises a longitudinal floor tunnel 100 forming arecess in the floor panel 24, opening downwardly. The floor tunnel 100extends from the front side 24 a of the floor panel 24 towards the rearside 24 b, between two side floor parts.

The floor tunnel 100 comprises a substantially horizontal upper wall 100a and two substantially longitudinal vertical side walls 100 b, 100 c.Each side wall 100 b, 100 c extends between an upper end, contiguous tothe upper wall, and a lower end, contiguous to one of the side floorparts.

The floor panel 24 is for example made of a steel.

The underbody reinforcement element 26 is attached to the lower sidefloor panel 24 and is destined to provide resistance to the vehicleunderbody structure 20.

The underbody reinforcement element 26 comprises a mesh of beamsdestined to absorb stress when the vehicle is subjected to an impact. Inparticular, the underbody reinforcement element 26 is destined to absorbstress received by a door ring, for example during a side impact.

The underbody reinforcement element 26 thus comprises severalreinforcement beams attached to the floor panel 24.

In particular, the reinforcement beams comprise at least a fronttransversal beam and a rear transversal beam, the front and reartransversal beams extending parallel to each other.

In the illustrated example, the reinforcement beams comprise two fronttransversal beams 112 a, 112 b and a single rear transversal beam 114.

The reinforcement beams further comprise two intermediate transversalbeams 116 a, 116 b, two longitudinal beams 118 a, 118 b and a tunneltransversal beam 120.

The two front transversal beams 112 a, 112 b extend transversally in thefront part of the floor panel 24, on either side of the floor tunnel100. Indeed, each front transversal beam 112 a, 112 b extends between aninner end contiguous to a lower end of a side wall 100 b, 100 c of thefloor tunnel 100 and an outer end contiguous to a side 24 c, 24 d of thefloor panel 24.

The rear transversal beam 114 extends in a central part of the floorpanel 24, parallel to the front transversal beams 112 a, 112 b. The rearfront transversal beam 114 thus extends between the right 24 c and left24 d sides of the floor panel 24.

More specifically, the front transversal beams 112 a, 112 b and the reartransversal beam 114 are configured so that, when the vehicle underbodystructure 20 is assembled to the door frame 22, the outer ends of eachfront transversal beam 112 a, 112 b may join the front pillar 38 of adoor frame 22, and each end of the rear transversal panel may join thecenter pillar 36 of a door frame 22.

The two intermediate transversal beams 116 a, 116 b extend between andparallel to the front transversal beams 112 a, 112 b and the reartransversal beam 114, on either side of the floor tunnel 100. Eachintermediate transversal beam 116 a, 116 b extends between an inner endcontiguous to a lower end of a side wall of the floor tunnel 100 and anouter end contiguous to a side of the floor panel 24.

For example, the intermediate transversal beams 116 a, 116 b extendsubstantially halfway between the front transversal beams 112 a, 112 band the rear transversal beam 114.

Thus, the intermediate transversal beams 116 a, 116 b are configured sothat, when the vehicle underbody structure 20 is assembled to the doorframe 22, the outer end of each intermediate transversal beam 116 a, 116b may join the lowerside sill 40 of a door frame 22.

The two longitudinal beams 118 a, 118 b extend longitudinally on eitherside of the floor tunnel 100. Each longitudinal beam 118 a, 118 bextends between a front end contiguous to a front transversal beam 112a, 112 b and a rear end contiguous to the rear transversal beam 114.Thus, each longitudinal beam 118 a, 118 b joins one of the fronttransversal beams 112 a, 112 b to the rear transversal beam 114.

Furthermore, each longitudinal beam 118 a, 118 b comprises anintermediate section which is contiguous to an inner end of anintermediate transversal beam 116 a, 116 b. Thus, each longitudinal beam118 a, 118 b joins one of the front transversal beams 112 a, 112 b to anintermediate transversal beam 116 a, 116 b and to the rear transversalbeam 114.

The tunnel transversal beam 120 extends from one longitudinal beam 118 ato the other longitudinal beam 118 b and across the floor tunnel 100.The tunnel transversal beam 120 thus comprises a center region 122crossing the floor tunnel 100 and comprised between two ends 124 a, 124b joining the longitudinal beams 118 a. The center region 120 a of thetunnel transversal beam 120 comprises a substantially horizontal upperwall, attached to the floor tunnel 100 upper wall, and two substantiallylongitudinal vertical side walls, each attached to a floor tunnel 100side wall.

Preferably, the tunnel transversal beam 120 is not transversally alignedwith the intermediate transversal beams 116 a, 116 b, so that alongitudinal offset exists between the tunnel transversal beam 120 andthe intermediate transversal beams 116 a, 116 b.

Owing to this offset, stress received by the intermediate transversalbeams 116 a, 116 b is not directly transmitted to the tunnel transversalbeam 120, but is transmitted through the longitudinal beams 118 a, 118b. Indeed, since the tunnel transversal beam 120 is not straight, itdoes not work in compression but in flexion when submitted to atransversal stress, and thus bends more easily than a straight beam. Theoffset between the tunnel transversal beam 120 and the intermediatetransversal beams 116 a, 116 b thus allows reducing the risks of bendingthe tunnel transversal beam 120.

Each front transversal beam 112 a, 112 b and the rear transversal beam114 are provided with slots 130 for receiving a connection element, alsocalled body mount, destined to connect the vehicle underbody structure20 to a chassis.

Preferably, the underbody reinforcement element 26 is made of a steel(or several different steels) different from the steel forming the floortunnel 100, and having a tensile strength higher than the steel formingthe floor tunnel 100.

Preferably, at least some of the reinforcement beams are made of a steelhaving a tensile strength higher than or equal to 1300 MPa. For example,the steel is a press hardening steel, which, once pressed, has a tensilestrength higher than or equal to 1300 MPa.

For example, the press-hardened steel has a composition comprising, in %weight, 0.10%≦C≦0.5%, 0.5%≦Mn≦3%, 0.1%≦Si≦1%, 0.01%≦Cr≦1%, Ti≦0.2%,Al≦0.1%, S≦0.05%, P≦0.1%, 0.0005%≦B≦0.010%, the remainder consisting ofiron and unavoidable impurities resulting from the production.

The press hardened steel is for example Usibor®, in particularUsibor®1500 or Usibor®2000.

The steel may be coated or uncoated, for example galvannealed orgalvanized by any suitable process such as hot dip coating,electrodeposition, vacuum coating.

In particular, the press hardened steel has preferably a structureconsisting essentially of ferrite and perlite before the steel ispressed, and a structure consisting essentially of martensite afterpressing.

The underbody reinforcement element 26 has an overall thickness, definedas the smallest dimension of the underbody reinforcement element 26, forexample comprised between 0.7 mm and 1.5 mm.

Preferably, at least one of the reinforcement beams has a thicknessdifferent from the thickness of the other beams. For example, thethicknesses of the front transversal beams 112 a, 112 b, the reartransversal beam 114, the intermediate transversal beams 116 a, 116 b,the longitudinal beams 118 a, 118 b and the tunnel transversal beam 120differ one from each other.

Each reinforcement beam has a thickness adapted to the position of thebeam and to the resistance to be achieved.

Preferably, the tunnel transversal beam 120 has a thickness higher thanthe other beams, due to the non straight geometry of this beam, inparticular in order to avoid bending of the tunnel transversal beam 120.

For example, the front transversal beams 112 a, 112 b have a thicknesssubstantially equal to 1.2 mm, and the rear transversal beam 114 has athickness substantially equal to 1.2 mm. The intermediate transversalbeams 116 a, 116 b have for example a thickness substantially equal to1.3 mm, the longitudinal beams 118 a, 118 b have for example a thicknesssubstantially equal to 1.2 mm, and the tunnel transversal beam 120 hasfor example a thickness substantially equal to 1.5 mm.

The reinforcement beams have an open cross section, so that theunderbody reinforcement element 26 forms an opened profile. The opencross-section of each reinforcement beam comprises at least a bottomsegment and two wall segments extending from either ends of the bottomsegment.

The underbody reinforcement element 26 is attached to the floor panel 24such that the floor panel 24 closes the opened profile.

Preferably, the underbody reinforcement element 26 and the floor panel24 are attached by welding, for example spot welding.

The production of the vehicle underbody structure 20 comprises theproduction of the floor panel 24, the production of the underbodyreinforcement element 26, and the assembling of the underbodyreinforcement element 26 to the floor panel 24 so as to form the vehicleunderbody structure 20.

The floor panel 24 is for example produced by stamping of asubstantially rectangular blank.

The underbody reinforcement element 26 is produced by forming andshaping each reinforcement beam and by assembling the reinforcementbeams so as to form the underbody reinforcement element 26.

The production of the underbody reinforcement element 26 thus comprisesa step of formation of substantially planar reinforcement beam blanks.

The formation of the reinforcement beam blanks comprises the formationof two front transversal beam blanks, a rear transversal beam blank, twointermediate transversal beam blanks, two longitudinal beam blanks and atunnel transversal beam blank.

The shapes and thicknesses of the reinforcement beam blanks are adaptedso that, once hot stamped, the front transversal beam blanks, the reartransversal beam blank, the intermediate transversal beam blanks, thelongitudinal beam blanks and the tunnel transversal beam blank may formthe front transversal beams 112 a, 112 b, the rear transversal beam 114,the intermediate transversal beams 116 a, 116 b, the longitudinal beams118 a, 118 b and the tunnel transversal beam 120 respectively.

The reinforcement beam blanks are for example obtained by cutting steelsheets, for example sheets made of a press hardening steel such asUsibor®, to the desired shapes.

The reinforcement beam blanks are then hot stamped so as to shape thereinforcement beams, and subsequently quenched.

The reinforcement beams are then assembled to form the underbodyreinforcement element 26.

In particular, the front end of each longitudinal beam 118 a, 118 b isassembled to the inner end of a front transversal beam 112 a, 112 b, anintermediate section of each longitudinal beam 118 a, 118 b is assembledto the inner end of an intermediate transversal beam 116 a, 116 b, andthe rear end of each longitudinal beam 118 a, 118 b is assembled to therear transversal beam 114.

Furthermore, each end of the tunnel transversal beam 120 is assembled toa longitudinal beam 118 a, 118 b.

Preferably, the reinforcement beams are assembled by means of welding,for example by spot welding or laser welding.

The use of a press hardening steel for at least part of the underbodyreinforcement element 26 provides both a good formability for thereinforcement beam blanks so that the reinforcement beam blanks may behot stamped to shape the reinforcement beams without obtainingcompression or thickening of the steel, and a ultra high strength forthe reinforcement beams once hot stamped and quenched.

The use of a press hardening steel having a tensile strength higher thanor equal to 1300 MPa provides an improved resistance to impacts, inparticular side impacts, without requiring thickening the beams, andthus without increasing the weight of the vehicle underbody structure.

The vehicle body 10 is formed by the assembly of the vehicle underbodystructure 20 and at least one vehicle door frame 22 on one side of thevehicle underbody structure 20, preferably two vehicle door frames 22 oneither side of the vehicle underbody structure 20. The assembling of thevehicle body 10 will now be described with a reference to one vehicledoor frame 22, on one side of the vehicle underbody structure 20, but itwill be understood that a second vehicle door frame 22 may be assembledin the same way to the other side of the vehicle underbody structure 20

The vehicle door frame 22 is attached to the vehicle underbody structure20 on a side of the vehicle underbody structure 20, preferably such thatthe front pillar 38 is attached to a front transversal beam 112 a of theunderbody reinforcement element 26 and the center pillar 36 is attachedto the rear transversal beam 114 of the underbody reinforcement element26. Furthermore, an intermediate transversal beam 116 a is attached tothe lowerside sill 40.

Thus, the energy of an impact received by the vehicle door frame 22, inparticular by the front and center pillars, can be efficientlytransmitted to the vehicle underbody structure 20.

Preferably, the transversal beams 112 a, 114 and 116 a are attached tothe vehicle door frame 22 by means of an inner lowerside sill part 140(FIG. 4), forming a joining steel sheet, which extends between the fronttransversal beam 112 a and the rear transversal beam 114. For example,the transversal beams 112 a, 114 and 116 a are attached to the vehicledoor frame 22 by welding.

The inner lowerside sill part 140 extends extends substantially along alongitudinal direction. The inner lowerside sill part 140 is assembled,for example by welding, to the outer lowerside sill part 80 to form thelowerside sill 40. The inner lowerside sill part 140 has a shapecomplementary to that of the outer lowerside sill part 80 such that,when the joining steel sheet 140 and the outer lowerside sill part 80are assembled, they form the lowerside sill 40, and such that thelowerside sill 40 has a hollow closed section.

The inner lowerside sill part 140 is for example made of a steel havinga fully martensitic microstructure, and having a tensile strengthpreferably higher than or equal to 1700 MPa. The steel forming the innerlowerside sill part 140 is for example MS1700®.

The outer lowerside sill part 80 is itself covered by an outer ornamentpanel 142.

Thus formed, the vehicle underbody structure 20 improves the resistanceof the vehicle in case of an impact, in particular a side impact. Inparticular, the position of the front and rear transversal beams, facingthe front pillar 38 and the center pillar 36 respectively, allowsdistributing the stress received by the vehicle during the impact, andprovides an efficient support to the front and center pillars.Furthermore, the intermediate transversal beam 116 a, facing thelowerside sill 40 may absorb energy in case of a side impact occurringbetween the front and center pillars, and thus limits the risks ofintrusion in the vehicle compartment in case of a side impact.

It must be understood that the example embodiments presented above arenot limiting.

What is claimed is: 1-16. (canceled)
 17. A vehicle underbody structurecomprising: a floor panel; and an underbody reinforcement element, saidunderbody reinforcement element comprising at least a front transversalbeam and a rear transversal beam, said front and rear transversal beamsextending parallel to each other and being attached to the floor panel,wherein the front transversal beam and the rear transversal beam aremade of a press hardening steel having a tensile strength higher than orequal to 1300 MPa.
 18. The vehicle underbody structure as recited inclaim 17 wherein the underbody reinforcement element further comprisesan intermediate transversal beam extending between and parallel to thefront transversal beam and the rear transversal beam, said intermediatetransversal beam being made of a press hardening steel having a tensilestrength higher than or equal to 1300 MPa.
 19. The vehicle underbodystructure as recited in claim 16 wherein the underbody reinforcementelement comprises at least one longitudinal beam extending from thefront transversal beam to the rear transversal beam, said longitudinalbeam being made of a press hardening steel having a tensile strengthhigher than or equal to 1300 MPa.
 20. The vehicle underbody structure asrecited in claim 16 wherein the floor panel comprises a longitudinalfloor tunnel forming a recess in the floor panel, the underbodyreinforcement element comprises a further front transversal beam, thefront transversal beam and the further front transversal beam extendingtransversally on either side of the floor tunnel.
 21. The vehicleunderbody structure as recited in claim 20 wherein the underbodyreinforcement element comprises two longitudinal beams, extending oneither side of the longitudinal floor tunnel, a first of said twolongitudinal beams extending from the front transversal beam to the reartransversal beam, and a second of said two longitudinal beams extendingfrom the further front transversal beam to the rear transversal beam,said longitudinal beams being made of a press hardening steel having atensile strength higher than or equal to 1300 MPa.
 22. The vehicleunderbody structure as recited in claim 21 wherein the underbodyreinforcement element comprises a tunnel transversal beam extending fromthe first of said two longitudinal beams to the second of said twolongitudinal beams and across the floor tunnel, said tunnel transversalbeam being made of a press hardening steel having a tensile strengthhigher than or equal to 1300 MPa.
 23. The vehicle underbody structure asrecited in claim 20 wherein the underbody reinforcement elementcomprises a single rear transversal beam extending transversally from atransversal end of the floor panel to the other transversal end of saidfloor panel at a longitudinal end of the floor tunnel.
 24. The vehicleunderbody structure as recited in claim 17 wherein the press hardeningsteel is Usibor®.
 25. The vehicle underbody structure as recited inclaim 17 wherein the underbody reinforcement element forms an openedprofile, said opened profile being closed by the floor panel.
 26. Thevehicle underbody structure as recited in claim 17 wherein the floorpanel is made of a material different than a material of the underbodyreinforcement element.
 27. The vehicle underbody structure as recited inclaim 17 further comprising at least one connection element of thevehicle underbody structure to a pick-up chassis, said connectionelement being attached to one of the front or rear transversal beams ofthe underbody reinforcement element.
 28. A vehicle body comprising: atleast a vehicle door frame; and a vehicle underbody structure as recitedin claim 17, wherein the vehicle door frame comprises at least a frontpillar, a center pillar and a side rail joining the front pillar and thecenter pillar, the front pillar being attached to the front transversalbeam of the underbody reinforcement element and the center pillar beingattached to the rear transversal beam of the underbody reinforcementelement.
 29. The vehicle body as recited in claim 28 wherein the doorframe further comprises a lowerside sill joining the front pillar to thecenter pillar, the front and rear transversal beams of the underbodyreinforcement element being welded to said lowerside sill.
 30. Thevehicle body as recited in claim 28 wherein the underbody reinforcementelement further comprises an intermediate transversal beam extendingbetween and parallel to the front transversal beam and the reartransversal beam, said intermediate transversal beam being made of apress hardening steel having a tensile strength higher than or equal to1300 MPa, the intermediate transversal beam being attached to thelowerside sill of the door frame.
 31. The vehicle body as recited inclaim 28 wherein the lowerside sill comprises an outer lowerside sillpart and an inner lowerside sill part, said front and rear transversalbeams of the underbody reinforcement element being welded to said innerlowerside sill part.
 32. The vehicle body as recited in claim 31 whereinsaid inner lowerside sill part is made of a steel having a fullymartensitic microstructure, said steel having a tensile strength higherthan or equal to 1700 MPa.
 33. The vehicle underbody structure asrecited in claim 17 wherein the floor panel comprises a longitudinalfloor tunnel forming a recess in the floor panel, and the underbodyreinforcement element comprises two intermediate transversal beams eachextending between and parallel to the front transversal beam and therear transversal beam on either side of the longitudinal floor tunnel, atunnel transversal beam extending across the floor tunnel, the tunneltransversal beam being transversally unaligned with the intermediatetransversal beams, so that a longitudinal offset exists between thetunnel transversal beam and the intermediate transversal beams.
 34. Thevehicle underbody structure as recited in claim 33 wherein the underbodyreinforcement element comprises two longitudinal beams, extending oneither side of the longitudinal floor tunnel, a first of said twolongitudinal beams extending from the front transversal beam to the reartransversal beam, and a second of said two longitudinal beams extendingfrom a further front transversal beam to the rear transversal beam, thetunnel transversal beam extending from the first longitudinal beam tothe second longitudinal beam.